MC-CDMA multiplexing in an orthogonal uplink

ABSTRACT

Techniques are provided to support multi-carrier code division multiple access (MC-CDMA) in an orthogonal uplink of a wireless communication system. A method of wireless multi-carrier communications comprises dividing sub-carriers on an uplink into non-overlapping groups, allocating a time-frequency block including a hopping duration and a non-overlapped group, respectively, assigning a different set of orthogonal codes to each user, spreading data (or pilot) symbols of each user over the allocated time-frequency block, wherein the data (or pilot) symbols of each user are spread using the different set of orthogonal codes assigned to each user, mapping each data (or pilot) symbol to a modulation symbol in the time-frequency block, generating an orthogonal waveform based on the mapped symbols, and transmitting the orthogonal waveform.

CROSS-REFERENCE TO RELATED APPLICATION Reference to Co-PendingApplications for Patent

The present Application for Patent is related to the followingco-pending U.S. Patent Application:

“Fast Frequency Hopping With a Code Division Multiplexed Pilot in anOFDMA System,” filed Dec. 3, 2003, patent application Ser. No.10/726,944, assigned to the assignee hereof, and expressly incorporatedby reference herein.

BACKGROUND

1. Field

The present invention relates generally to communication, and morespecifically to techniques for supporting multi-carrier code divisionmultiple access (MC-CDMA) in an orthogonal uplink of a wirelesscommunication system.

2. Background

In a frequency hopping spread spectrum (FHSS) communication system, datais transmitted on different frequency subbands or sub-carriers indifferent time intervals, which are also referred to as “hop periods”.These frequency subbands may be provided by orthogonal frequencydivision multiplexing (OFDM), other multi-carrier modulation techniques,or some other constructs. With FHSS, the data transmission hops fromsubband to subband in a pseudo-random manner. This hopping providesfrequency diversity and allows the data transmission to better withstanddeleterious path effects such as narrow-band interference, jamming,fading, and so on.

An OFDMA system utilizes OFDM and can support multiple userssimultaneously. For a frequency hopping OFDMA system, data for each useris transmitted using a specific frequency hopping (FH) sequence assignedto the user. The FH sequence indicates the specific subband to use fordata transmission in each hop period. Multiple data transmissions formultiple users may be sent simultaneously using different FH sequences.These FH sequences are defined to be orthogonal to one another so thatonly one data transmission uses each subband in each hop period. Byusing orthogonal FH sequences, intra-cell interference is avoided, andthe multiple data transmissions do not interfere with one another whileenjoying the benefits of frequency diversity.

SUMMARY

Techniques are provided herein to support MC-CDMA multiplexing in anorthogonal uplink of a wireless communication system.

In an aspect, a method of wireless multi-carrier communications,comprises dividing sub-carriers on an uplink into non-overlappinggroups, allocating at least one time-frequency block, eachtime-frequency block having a hopping duration and a non-overlappedgroup, assigning a different set of orthogonal codes to each user,spreading symbols of each user over the allocated at least onetime-frequency block, wherein the symbols of each user are spread usingthe different set of orthogonal codes assigned to each user, mappingeach symbol to a modulation symbol in the at least one time-frequencyblock, generating an orthogonal waveform based on the mapped symbols;and transmitting the orthogonal waveform.

In an aspect, the orthogonal waveform generated is an orthogonalfrequency division multiple (OFDM) waveform. In another aspect, theorthogonal waveform generated is an orthogonal frequency divisionmultiple access (OFDMA) waveform.

In an aspect, an apparatus for wireless multi-carrier communicationscomprises means for dividing sub-carriers on an uplink intonon-overlapping groups, means for allocating at least one time-frequencyblock, each time-frequency block having a hopping duration and anon-overlapped group, means for assigning a different set of orthogonalcodes to each user, means for spreading symbols of each user over theallocated at least one time-frequency block, wherein the symbols of eachuser are spread using the different set of orthogonal codes assigned toeach user, means for mapping each symbol to a modulation symbol in theat least one time-frequency block, means for generating an orthogonalwaveform based on the mapped symbols, and means for transmitting theorthogonal waveform.

In yet another aspect, a computer readable media embodying a method forwireless multi-carrier communications comprises dividing sub-carriers onan uplink into non-overlapping groups, allocating at least onetime-frequency block, each time-frequency block having a hoppingduration and a non-overlapped group, assigning a different set oforthogonal codes to each user, spreading symbols of each user over theallocated at least one time-frequency block, wherein the symbols of eachuser are spread using the different set of orthogonal codes assigned toeach user, mapping each symbol to a modulation symbol in the at leastone time-frequency block, generating an orthogonal waveform based on themapped symbols, and transmitting the orthogonal waveform.

In still yet another aspect, an apparatus for wireless multi-carriercommunications comprises a controller, a processor, and a transmitter.The controller is operative to divide sub-carriers on an uplink intonon-overlapping groups, allocate at least one time-frequency block, eachtime-frequency block having a hopping duration and a non-overlappedgroup, and assign a different set of orthogonal codes to each user. Theprocessor is operative to spread symbols of each user over the allocatedat least one time-frequency block, wherein the symbols of each user arespread using the different set of orthogonal codes assigned to each userand map each symbol to a modulation symbol in the at least onetime-frequency block. The transmitter is operative to generate anorthogonal waveform based on the mapped symbols, and transmit theorthogonal waveform.

In an aspect, a receiver in a wireless multi-carrier communicationssystem comprises an antenna for receiving an orthogonal waveform, ademodulator for demodulating the orthogonal waveform, thereby creatingspread symbols, a processor for determining a time-frequency block fromthe spread symbols, and a de-spreader for despreading the spread symbolsin the time-frequency block using an orthogonal code for a user.

Various aspects and embodiments of the invention are described infurther detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, nature, and advantages of the present invention willbecome more apparent from the detailed description set forth below whentaken in conjunction with the drawings in which like referencecharacters identify correspondingly throughout and wherein:

FIG. 1 illustrates the concept of MC-CDMA in the context of FH-OFDMA inaccordance with an embodiment;

FIG. 2 shows a block diagram of a terminal in accordance with anembodiment; and

FIG. 3 shows a block diagram a base station in accordance with anembodiment.

DETAILED DESCRIPTION

The word “exemplary” is used herein to mean “serving as an example,instance, or illustration.” Any embodiment or design described herein as“exemplary” is not necessarily to be construed as preferred oradvantageous over other embodiments or designs.

An OFDMA system may be deployed with multiple cells, where a celltypically refers to a base station and/or its coverage area. A datatransmission on a given subband in one cell acts as interference toanother data transmission on the same subband in a neighboring cell. Torandomize inter-cell interference, the FH sequences for each cell aretypically defined to be pseudo-random with respect to the FH sequencesfor neighboring cells. By using pseudo-random FH sequences, interferencediversity is achieved, and the data transmission for a user in a givencell would observe, over a sufficiently long time period, the averageinterference from the data transmissions for other users in other cells.

The inter-cell interference can vary significantly from subband tosubband at any given moment. To account for the variation ininterference across the subbands, a margin is typically used in theselection of a data rate for a data transmission. A large margin isnormally needed to achieve a low packet error rate (PER) for the datatransmission if the variability in interference is large. The largemargin results in a greater reduction in the data rate for the datatransmission, which limits system capacity.

Frequency hopping can average the inter-cell interference and reduce therequired margin. Increasing the frequency hopping rate results in betterinterference averaging and decreases the required margin. Fast frequencyhopping rate is especially beneficial for certain types of transmissionsthat encode data across multiple frequency hops and which cannot useother techniques, such as automatic request for retransmission (ARQ), tomitigate the deleterious effects of interference.

Multi-Carrier Code Division Multiple Access (MC-CDMA) system with anFH-OFDMA uplink is a communication system based on a combination of CDMAscheme and orthogonal frequency division multiplexing (OFDM) signaling.MC-CDMA is an effective transmission technique on the downlink, as theorthogonality between multiplexed signals can still be preserved evenafter going through a multi-path channel (assuming accurate time andfrequency synchronization between users and a base station), therebyallowing reliable separation of the multiplexed signals at the receiver.

On the other hand, MC-CDMA hasn't been as successful as amultiple-access technique on the uplink. The uplink transmission isinherently different from the downlink transmission in that transmittedsignals from different users are affected by different channels. Due tothe nature of multiplexing and sensitivity to channel estimation errorof MC-CDMA, a disproportionate amount of system resource must be setaside for channel estimation in order for this technique to work on theuplink. Furthermore, synchronization on the uplink is a more complexproblem since users see different channels, Doppler shifts, and areoften at different distances from the base station.

However, a careful application of MC-CDMA as a multiplexing technique inthe context of an FH-OFDMA uplink can result in significant resourceutilization improvement, especially in terms of bandwidth utilizationfor low-spectral efficiency transmission.

In FH-OFDMA a user on the uplink is assigned a subset of sub-carriersand hops over time. Hopping helps improve frequency diversity andinterference averaging over time. In an embodiment, sub-carriers on theuplink are divided into non-overlapping groups and each group hopsindependently. Since channels from consecutive sub-carriers (within agroup) are expected to be highly correlated, their channels can beestimated using common pilot symbols, which leads to a significantsaving on the pilot overhead (compared to a deployment with randomsub-carrier hopping). Furthermore, FH-OFDMA employs a closed-loop uplinktime-control mechanism to ensure that all uplink signals arrive within asmall time window (i.e., within a cyclic prefix duration), which helpsfacilitate inter-symbol interference (ISI) and inter-carrierinterference (ICI) mitigation.

In an embodiment, FH-OFDMA supports MC-CDMA multiplexing either acrossdifferent users or across different signals from the same user. FIG. 1illustrates the concept of MC-CDMA in the context of FH-OFDMA inaccordance with an embodiment. The horizontal axis is for OFDM symbols102. The vertical axis is for sub-carriers 104.

The example assumes 8-carrier group hopping over 8 OFDM symbols. Assuch, there are 64 modulation symbols in each time-frequency block 106.Hop duration 108 and carrier association 110 are shown for atime-frequency block.

In an embodiment, the time and frequency are contiguous in atime-frequency block. A time-frequency block is a contiguous allocationof OFDM symbols and subcarriers. Alternatively, frequency is notcontiguous in a time-frequency block, but frequencies that are part ofthe same time-frequency block are orthogonal to each other.

Each user is assigned a different set of orthogonal codes to be used inspreading respective data (or pilot) symbols over the allocatedtime-frequency block. Examples of orthogonal codes include Walsh codesand Gold codes, which are both known in the art.

After spreading, each symbol is mapped to one of the modulation symbolsin the assigned time-frequency block. A corresponding OFDMA waveform isthen generated based on these symbols (following standard OFDMA waveformgeneration technique). As can be seen, multiple users are sharing thesame time-frequency allocation-a marked distinction from the traditionalFH-OFDMA where users are assigned different sets of time-frequencyallocation to ensure orthogonality. With proper choices of number ofsub-carriers in a group and hop duration, respective channels ofdifferent users appear to be constant over certain time-frequencyallocation, thereby allowing users to be separated based on the uniquespreading signatures/codes assigned to different users.

MC-CDMA signals from different users are multiplexed over the sametime-frequency allocation. A respective channel from each user isexpected to be constant over each time-frequency allocation, therebyallowing separation at the receiver.

This technique is particularly effective in multiplexing low-spectralefficiency transmissions from different users (e.g., pilot symbols,ACK/NACK symbols, etc.) over the same time-frequency allocation on theuplink. Furthermore, this technique can also be used to help alleviatelink budget constraint in certain scenarios.

As an example, a one-bit transmission (e.g., a pilot or ACK/NACK symbol)on the uplink is considered. In order to meet the performancerequirement, a certain amount of received SNR must be achieved. A usercan either transmit the bit over one transmission at a very high poweror transmit at a lower power over several transmissions (e.g., throughrepetition). The former technique results in high bandwidth efficiency(i.e., only one transmission is required) but may suffer from linkbudget constraint and, worse yet, from poor performance due to lack offrequency/interference diversity. The alternative approach is totransmit this one bit over several transmissions. To improvefrequency/interference diversity each transmission may take place overdifferent frequency and/or time instants. This approach will likelyresult in more reliable detection at the receiver, but this is at theexpense of larger bandwidth overhead and possibly longer transmissiontime. Longer transmission time of an ACK/NACK bit results in lessprocessing time at the transmitter, especially in a system where H-ARQis used.

A compromise is to use a transmission technique that is able to garnersufficient amount of frequency/interference diversity while still usingreasonable amount of bandwidth. A structure with consecutive carriergroup hopping considered previously can be used. In this setting, a usertransmits the one-bit quantity over multiple time-frequency blocks inorder collect frequency/interference diversity. Furthermore, multipleusers are orthogonally multiplexed over a particular time-frequencyblock to minimize the overall bandwidth consumption. To see this latterpoint, consider a scenario where a user transmits the one-bit quantityover M transmissions. Assume that N transmissions fall within aparticular time-frequency block (i.e., a user transmits over a total ofM/N blocks). As such, a user requires N modulation symbols from eachblock. Assuming that there are a total of K modulation symbols pertime-frequency block, each block can then support at most K/N users.Clearly, if the channel remains fairly constant (in both time andfrequency) over each time-frequency block, one can readily apply theMC-CDMA multiplexing technique. Towards that end, each user is assignedone of the orthogonal code sequences to modulate a respective datasymbol. Orthogonally-spread symbols are then put on appropriatesub-carriers from which an OFDM waveform can be generated.

By assigning an orthogonal code sequence to each user, up to K users canbe multiplexed in each time-frequency block while still being able tocollect the same amount of energy (after despreading). In addition,since each user is now transmitting over the entire time-frequencyblock, a saving on the link budget is an immediate byproduct. The linkbudget saving comes primarily from the fact that each user istransmitting over a longer duration.

This transmission technique can also be generalized to work in a settingwhere users are transmitting more than just one bit each. In particular,it is always possible to modify the transmission of each user such thatmultiple users can be multiplexed over each time-frequency block (i.e.,through a deliberate spreading). The true bandwidth saving, however,really comes when repetition code (which is a form of spreading) isinherent to the transmission.

In an FH-OFDMA setting, a repetition code is also useful as a means toalleviate link budget constraint. For instance, due to a link budgetlimitation a user may not be able to meet the received SNR requirementwhen transmitting a coded symbol over one transmission. One way ofgetting around this is to transmit each coded symbol over multipletransmissions, each with lower power, at different time instants (i.e.,through repetitions). Clearly, by applying the proposed usermultiplexing technique, the desired result can be achieved whilelimiting the bandwidth overhead to a minimum.

FIG. 2 shows a block diagram of an embodiment of a terminal 220 x, whichis one of the terminals in OFDMA system 200. For simplicity, only thetransmitter portion of terminal 220 x is shown in FIG. 2.

Within terminal 220 x, an encoder/interleaver 212 receives traffic datafrom a data source 210 and possibly control data and other data from acontroller 240. Encoder/interleaver 212 formats, encodes, andinterleaves the received data to provide coded data. A modulator 214then modulates the coded data in accordance with one or more modulationschemes (e.g., QPSK, M-PSK, M-QAM, and so on) to provide modulationsymbols (or simply, “data symbols”). Each modulation symbol is a complexvalue for a specific point in a signal constellation for the modulationscheme used for that modulation symbol.

An OFDM modulator 220 performs frequency hopping and OFDM processing forthe data symbols. Within OFDM modulator 220, a TX FH processor 222receives the data symbols and provides these data symbols on the propersubbands determined by an FH sequence for a traffic channel assigned toterminal 220 x. This FH sequence indicates the specific subband to usein each hop period and is provided by controller 240. The TX FHprocessor 222 provides data symbols. The data symbols dynamically hopfrom subband to subband in a pseudo-random manner determined by the FHsequence. For each OFDM symbol period, TX FH processor 222 provides N“transmit” symbols for the N subbands. These N transmit symbols arecomposed of one data symbol for the subband used for data transmission(if data is being transmitted) and a signal value of zero for eachsubband not used for data transmission.

An inverse fast Fourier transform (IFFT) unit 224 receives the Ntransmit symbols for each OFDM symbol period. IFFT unit 224 thentransforms the N transmit symbols to the time domain using an N-pointinverse FFT to obtain a “transformed” symbol that contains N time-domain“data” chips. Each data chip is a complex value to be transmitted in onechip period. (The chip rate is related to the overall bandwidth of thesystem.) A cyclic prefix generator 226 receives the N data chips foreach transformed symbol and repeats a portion of the transformed symbolto form an OFDM symbol that contains N+C_(p) data chips, where C_(p) isthe number of data chips being repeated. The repeated portion is oftenreferred to as a cyclic prefix and is used to combat inter-symbolinterference (ISI) caused by frequency selective fading. An OFDM symbolperiod corresponds to the duration of one OFDM symbol, which is N+C_(p)chip periods. Cyclic prefix generator 226 provides a stream of datachips for a stream of OFDM symbols.

A transmit (TX) pilot processor 230 receives the stream of data chipsand at least one pilot symbol. TX pilot processor 230 generates anarrowband pilot. TX pilot processor 230 provides a stream of “transmit”chips. A transmitter unit (TMTR) 232 processes the stream of transmitchips to obtain a modulated signal, which is transmitted from an antenna234 to the base station.

FIG. 3 shows a block diagram of an embodiment of a base station 210 x,which is one of the base stations in OFDMA system 200. For simplicity,only the receiver portion of base station 210 x is shown in FIG. 3.

The modulated signal transmitted by terminal 220 x is received by anantenna 252. The received signal from antenna 252 is provided to andprocessed by a receiver unit (RCVR) 254 to provide samples. Receiverunit 254 may further perform sample rate conversion (from the receiversampling rate to the chip rate), frequency/phase correction, and otherpre-processing on the samples. Receiver unit 254 provides a stream of“received” chips.

A receive (RX) pilot processor 260 receives and processes the stream ofreceived chips to recover the narrowband pilot and the data chipstransmitted by terminal 220 x. Several designs for RX pilot processor260 are described below. RX pilot processor 260 provides a stream ofreceived data chips to an OFDM demodulator 270 and channel gainestimates to a digital signal processor (DSP) 262. DSP 262 processes thechannel gain estimates to obtain channel response estimates used fordata demodulation, as described below.

Within OFDM demodulator 270, a cyclic prefix removal unit 272 receivesthe stream of received data chips and removes the cyclic prefix appendedto each received OFDM symbol to obtain a received transformed symbol. AnFFT unit 274 then transforms each received transformed symbol to thefrequency domain using an N-point FFT to obtain N received symbols forthe N subbands. An RX FH processor 276 obtains the N received symbolsfor each OFDM symbol period and provides the received symbol from theproper subband as the received data symbol for that OFDM symbol period.The specific subband from which to obtain the received data symbol ineach OFDM symbol period is determined by the FH sequence for the trafficchannel assigned to terminal 220 x. This FH sequence is provided by acontroller 290. Since the data transmission by terminal 220 xdynamically hops from subband to subband, RX FH processor 276 operatesin unison with TX FH processor 222 in terminal 220 x and provides thereceived data symbols from the proper subbands. The FH sequence used byRX FH processor 276 at base station 210 x is the same as the FH sequenceused by TX FH processor 222 at terminal 220 x. Moreover, the FHsequences at base station 210 x and terminal 220 x are synchronized. RXFH processor 276 provides a stream of received data symbols to ademodulator 280.

Demodulator 280 receives and coherently demodulates the received datasymbols with the channel response estimates from DSP 262 to obtainrecovered data symbols. The channel response estimates are for thesubbands used for data transmission. Demodulator 280 further demaps therecovered data symbols to obtain demodulated data. Adeinterleaver/decoder 282 then deinterleaves and decodes the demodulateddata to provide decoded data, which may be provided to a data sink 284for storage. In general, the processing by the units in base station 210x is complementary to the processing performed by the correspondingunits in terminal 420 x.

Controllers 240 and 290 direct operation at terminal 220 x and basestation 210 x, respectively. Memory units 242 and 292 provide storagefor program codes and data used by controllers 240 and 290,respectively. Controllers 240 and 290 may also perform pilot-relatedprocessing. For example, controllers 240 and 290 may determine the timeintervals when a narrowband pilot for terminal 220 x should betransmitted and received, respectively.

For clarity, FIGS. 2 and 3 show transmission and reception,respectively, of pilot and data on the reverse link. Similar ordifferent processing may be performed for pilot and data transmission onthe forward link.

The techniques described herein may be used for a frequency hoppingOFDMA system as well as other wireless multi-carrier communicationsystems. For example, these techniques may be used for systems thatemploy other multi-carrier modulation techniques such as discretemulti-tone (DMT).

The techniques described herein may be used for efficient narrowbanduplink pilot transmissions in a Time Division Duplexing (TDD)deployment. The saving is in both system bandwidth and link budget foreach user. For example, given three users, each transmitting a symbolover three time slots, each user transmits its symbol at ⅓ transmissionpower over three time slots.

The techniques described herein may be implemented by various means atthe transmitter and the receiver. The pilot and data processing at thetransmitter and receiver may be performed in hardware, software, or acombination thereof. For a hardware implementation, the processing units(e.g., TX pilot processor 230, RX pilot processor 260, DSP 222, and soon) may be implemented within one or more application specificintegrated circuits (ASICs), digital signal processors (DSPs), digitalsignal processing devices (DSPDs), programmable logic devices (PLDs),field programmable gate arrays (FPGAs), processors, controllers,micro-controllers, microprocessors, other electronic units designed toperform the functions described herein, or a combination thereof.

For a software implementation, the pilot and data processing at thetransmitter and receiver may be implemented with modules (e.g.,procedures, functions, and so on) that perform the functions describedherein. The software codes may be stored in memory units (e.g., memoryunits 242 and 292 in FIGS. 2 and 3) and executed by processors (e.g.,controllers 240 and 290). The memory unit may be implemented within theprocessor or external to the processor, in which case it can becommunicatively coupled to the processor via various means as is knownin the art.

The previous description of the disclosed embodiments is provided toenable any person skilled in the art to make or use the presentinvention. Various modifications to these embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without departing from thespirit or scope of the invention. Thus, the present invention is notintended to be limited to the embodiments shown herein but is to beaccorded the widest scope consistent with the principles and novelfeatures disclosed herein.

1. A method of wireless multi-carrier communications, comprising:dividing sub-carriers on an uplink into non-overlapping groups;allocating at least one time-frequency block, each time-frequency blockhaving a hopping duration and a non-overlapped group; assigning adifferent set of orthogonal codes to each user; spreading symbols ofeach user over the allocated at least one time-frequency block, whereinthe symbols of each user are spread using the different set oforthogonal codes assigned to each user; mapping each spread symbol to amodulation symbol in the at least one time-frequency block; generatingan orthogonal waveform based on the mapped symbols; and transmitting theorthogonal waveform.
 2. The method of claim 1, wherein dividingsub-carriers further comprises dividing contiguous sub-carriers into anon-overlapping group.
 3. The method of claim 1, wherein generating anorthogonal waveform comprises generating an orthogonal frequencydivision multiple (OFDM) waveform, and wherein transmitting theorthogonal waveform comprises transmitting an OFDM waverform.
 4. Themethod of claim 1, wherein generating an orthogonal waveform comprisesgenerating an orthogonal frequency division multiple access (OFDMA)waveform, and wherein transmitting the orthogonal waveform comprisestransmitting an OFDMA waverform.
 5. The method of claim 1, wherein thenon-overlapped groups hop independently.
 6. The method of claim 1,wherein spreading symbols comprises spreading pilot symbols.
 7. Themethod of claim 1, wherein spreading symbols comprises spreadingACK/NACK symbols.
 8. The method of claim 1, wherein spreading symbolscomprises spreading CQI symbols.
 9. The method of claim 1, whereinspreading symbols comprises spreading Request symbols.
 10. The method ofclaim 1, wherein the orthogonal codes are Walsh codes.
 11. The method ofclaim 1, wherein the orthogonal codes are Gold codes.
 12. An apparatusfor wireless multi-carrier communications, comprising: means fordividing sub-carriers on an uplink into non-overlapping groups; meansfor allocating at least one time-frequency block, each time-frequencyblock having a hopping duration and a non-overlapped group; means forassigning a different set of orthogonal codes to each user; means forspreading symbols of each user over the allocated at least onetime-frequency block, wherein the symbols of each user are spread usingthe different set of orthogonal codes assigned to each user; means formapping each spread symbol to a modulation symbol in the at least onetime-frequency block; means for generating an orthogonal waveform basedon the mapped symbols; and means for transmitting the orthogonalwaveform.
 13. An apparatus for wireless multi-carrier communications,comprising: a controller operative to: divide sub-carriers on an uplinkinto non-overlapping groups, allocate at least one time-frequency block,each time-frequency block having a hopping duration and a non-overlappedgroup; and assign a different set of orthogonal codes to each user; aprocessor operative to: spread symbols of each user over the allocatedat least one time-frequency block, wherein the symbols of each user arespread using the different set of orthogonal codes assigned to eachuser; and map each symbol to a modulation symbol in the at least onetime-frequency block; and a transmitter operative to: generate anorthogonal waveform based on the mapped symbols; and transmit theorthogonal waveform.
 14. A receiver in a wireless multi-carriercommunications system, comprising: an antenna for receiving anorthogonal waveform; a demodulator for demodulating the orthogonalwaveform, thereby creating spread symbols; a processor for determining atime-frequency block from the spread symbols; and a de-spreader forde-spreading the spread symbols in the time-frequency block using anorthogonal code for a user.
 15. A readable media embodying a method forwireless multi-carrier communications, the method comprising: dividingsub-carriers on an uplink into non-overlapping groups; allocating atleast one time-frequency block, each time-frequency block having ahopping duration and a non-overlapped group; assigning a different setof orthogonal codes to each user; spreading symbols of each user overthe allocated at least one time-frequency block, wherein the symbols ofeach user are spread using the different set of orthogonal codesassigned to each user; mapping each spread symbol to a modulation symbolin the at least one time-frequency block; generating an orthogonalwaveform based on the mapped symbols; and transmitting the orthogonalwaveform.